UNC

TgUNC is a UCS-family myosin co-chaperone in Toxoplasma gondii that is dedicated to the folding, assembly, and function of the parasite's myosins (PMID:28593938, PMID:25231988). In a heterologous Sf9 expression system, co-expression of TgUNC is required to produce soluble, functional TgMyoA (class XIVa myosin), and the resulting myosin is competent for actin-based gliding motility in vitro; the tetratricopeptide repeat (TPR) domain of TgUNC is dispensable for generating functional myosin, indicating that Hsp90 recruitment occurs through other means (PMID:25231988). Conditional ablation of TgUNC in the parasite recapitulates the combined phenotypes of all known myosins, producing defects in motility, invasion, egress, organelle positioning, basal complex constriction, and synchronized intravacuolar division, establishing TgUNC as a shared chaperone upon which the entire myosin repertoire depends (PMID:28593938).

1. 2014 High

   Establishing whether Toxoplasma myosins require a dedicated co-chaperone to fold correctly, this work showed that TgUNC is needed to produce soluble, motility-competent TgMyoA and that its TPR domain is not required for that activity.

   Evidence Baculovirus/Sf9 heterologous co-expression with in vitro actin gliding motility assay and TPR-domain deletion mutagenesis

   PMID:25231988

   Open questions at the time

   • How Hsp90 is recruited if not via the TPR domain was not resolved
   • Tested on TgMyoA in a heterologous system; whether other parasite myosins depend on TgUNC was not addressed here
   • No in vivo requirement demonstrated in the parasite
2. 2017 High

   Extending the co-chaperone role from a single myosin in vitro to the whole parasite, conditional ablation showed TgUNC is required for the function of all Toxoplasma myosins, recapitulating the entire myosin phenome.

   Evidence Conditional knockdown/ablation in the parasite with motility, invasion, egress, basal complex constriction, and division-synchrony readouts

   PMID:28593938

   Open questions at the time

   • Direct biochemical demonstration that each individual myosin physically depends on TgUNC was not shown for every myosin
   • Molecular mechanism of the folding/assembly cycle (client engagement, ATP dependence) not resolved
   • Single lab; comprehensive but not independently replicated

• How TgUNC engages Hsp90 in the absence of a required TPR domain, and the structural basis of client myosin folding, remain unresolved.
• Mechanism of Hsp90 recruitment independent of the TPR domain is unknown
• No structural model of the TgUNC-myosin chaperone complex
• Client-specificity determinants across the myosin repertoire not defined